The invention relates to methods of testing insulated underground power cables for degradation of the insulation, and more particularly to on-line diagnostic methods.
Electrical power is typically distributed throughout the world using insulated underground power cables. Such power cables typically consist of a conductive core of a bundle of conducting strands, surrounded by a semi-conducting shield layer, an insulation layer, a second semi-conducting shield layer, a layer of metallic tape or helical concentric neutral conducting strands, and a polymeric jacket or sheath. The insulation on such power cables has a typical lifetime of about 30 to 40 years. However various factors can cause premature degradation of the insulation and resultant failure of the cable. One typical form of degradation is xe2x80x9cwater treeingxe2x80x9d. Polymer insulation absorbs moisture over time, and collections of moisture in the power cable insulation under electrical stress are referred to as xe2x80x9cwater treesxe2x80x9d. Such water trees can cause capacitive leakage current distortion from the central conductor which over time and under certain electrical conditions can lead to electrical treeing and ultimately complete failure of the cable. Similarly, partial discharges and electrical treeing can occur due to the presence of imperfections such as cavities or contaminants or particles in the insulation layer within the cable or its accessories. Such treeing can cause premature failure of the power cables.
It is important therefore for utilities to be able to test power cables for degradation to allow replacement prior to failure and to permit an orderly replacement schedule. Preferably such testing would be done xe2x80x9clivexe2x80x9d, that is without de-energising the cable prior to testing. While the Japanese power system which uses ungrounded delta connected cables has permitted live testing because of its different grounding arrangement, North American power cables and in most other countries world-wide, use grounded, star connected cables that have required in the past that the cable be de-energising for testing. There is therefore a need for a diagnostic method which permits on-line, non-destructive live diagnostics in the North American and world-wide cable systems. While various on and off line diagnostics are known, they generally require different methods of detecting the current leakage, and consequently it is not possible to carry out multiple diagnostics using the same measurement set-up.
The invention provides a method of on-line diagnosis of insulation degradation in a portion of underground cable comprising:
i) measuring the current flow at first and second locations of the cable utilizing an optical current sensor at one of a plurality of current frequencies:
ii) determining the current flow differential at said one frequency;
iii) analysing the results of the current flow differential determination according to one or more of the following diagnostic analyses to assess the need for cable replacement:
a) a corrosion test on the cable to ensure the safety of conducting tests thereon;
b) a partial discharge analysis; and
c) one of the following two diagnostics, depending on the type of insulation material:
i) a harmonic current analysis if the insulation layer is cross-linked polyethylene or ethylene propylene rubber, or
ii) a tangent delta or loss angle diagnostic if the insulation layer is paper insulated lead covered or ethylene propylene rubber.